Pore structure and permeability of tectonically deformed coals related to underground gasification from the Huaibei-Huainan coalfield

Yiwen Ju, Graduate University of Chinese Academy of Sciences (China)Quanlin Hou, Graduate University of Chinese Academy of Sciences (China)Xiaoshi Li, Graduate University of Chinese Academy of Sciences (China)

Large quantities of coal are available and gasification is seen as part of a solution to global energy deficiency. Gasification is of particular value in areas where the coal measures may not be extractable, i.e. too deep, or complicated tectonically deformed coalbeds. Porosity, pore diameter distribution, pore specific surface area and their permeability are the principal physical properties of coal gasification. Tectonic deformation can change the molecular structure and chemical composition of coal. It can also influence pore structure, fracture and permeability. On the basis of coal fracture observation, we have studied the pore structure of twelve sets of tectonically deformed coals with mercury intrusion and low-temperature nitrogen adsorption measurements. Coal samples were collected from 11 coal mines in the Huaibei-Huainan coalfield, China. In this study we also tested the coals' permeability using simulated stress and pressure conditions in situ. The results indicated that, for the different types of tectonically deformed coals formed in the metamorphic and deformational environments of low-rank coal, it was determined that they had a greater volume and specific surface area of medium pore (100-2000 nm) than normal coal, and mesopore(15-100 nm) decreased rapidly with increasing tectonic deformation, while micropore(5-15 nm), sub-micropore (2.5-5 nm) and ultra-micropore (<2.5 nm) increased. Similar phenomena were observed in the tectonically deformed coals formed in the metamorphic and deformational environments of medium- and high-rank coal. Tectonically formed coals possess three to more than ten times more porosity and two to more than ten times more specific surface area than their normal counterparts. Moreover, the occurrence of a large number of the pores which the pore diameter exceeds micropores and tectonic fractures in a formed coal zone will make it easier to increase the permeability of coal, and the permeability of brittle-deformation coal is higher than that of ductile-deformation coal. In addition, for low ductile deformation superposed by late brittle deformation, permeability increases with deformation. Therefore, the gasification of low metamorphic and brittle-deformed coals or low metamorphic and ductile deformation superposed by late brittle deformation is much easier than the others.